The invention relates to a distributed threshold voltage field effect transistor (DTV-FET) wherein a first field effect transistor (FET) and a second FET are coupled in series and a voltage is applied to the gate of the first FET.
A low off-current is a very important factor for amorphous-silicon field-effect transistors (a-Si FETs) used in active matrices. This is because a high off-current causes various problems in FET-addressed liquid-crystal displays, such a degradation of optical image quality.
The Distributed Threshold Voltage Field Effect Transistor (DTV-FET) was recently proposed (Uchida and Matsumura, Jpn. J Appl Phys 27 L2379 (1988)) under these circumstances. It incorporates a structured channel of amorphous-silicon (a-Si:H) or poly-silicon such that the channel has a changing local threshold voltage Vt from source to drain. The effect of this is to increase the separation (in gate voltage terms) between electron conduction and hole conduction. This increased separation is called the DTV-FET effect. The advantage of this effect is that it allows a larger choice of voltages ranges that can be used, giving greater design freedom. This freedom can be used for instance to counter a non-uniform distribution of transistor characteristics within an active matrix, or to increase the choice of liquid crystals that can be used. Also, the reduction in off current that occurs as a result of the DTV-FET effect leads to an improved performance of active matrix LCD displays.
The simplest way to realize the DTV-FET is, as shown in FIG. 8, to externally connect two normal transistors in series and apply the gate voltage Vg plus a constant offset voltage Vos to the transistor on the drain side.
A second method to fabricate the DTV-FET is, as shown in FIG. 9a, to dope part of the channel. The channel region denoted "n.sup.- a-Si" is nominally undoped, but the region denoted "n a-Si" is moderately doped. At moderate doping levels, the effect of the doping is to shift the characteristics (in gate voltage terms) without altering their shape. Hence the threshold voltage distribution is a step function along the channel. This then results in the DTV-FET effect.
FIG. 9b shows an equivalent circuit of the DTV-FET shown in FIG. 9a which is formed by connecting two uniform FETs, Q.sub.s and Q.sub.d, in series. FIG. 9c shows one example of off-characteristics of DTV-FET shown in FIG. 9a wherein the dotted curves are those of the individual uniform FETs.
As outlined above, the DTV-FET structures proposed so far achieve the DTV-FET effect either by using an extra offset voltage as DTV-FET in FIG. 8 or by doping part of the channel as one in FIG. 9a.
The disadvantage of the first method is that it requires application of at least a third voltage to what is in effect a four terminal device. This means that in the circuit in which the transistor is incorporated (for example, the liquid crystal display's active matrix) additional scan lines have to be used to lead the extra voltage to the transistor. The fabrication of these additional scan lines increases production cost an their presence increases the chance of display failure due to, for example, cross-over shorts. Furthermore, the area occupied by the additional scan lines reduces the display's aperture ratio.
The second method of creating the DTV-FET by doping part of the channel avoids the need for an extra voltage and the problem of the additional scan lines. However, the amplitude of the distribution in VT and therefore the magnitude of the DTV-FET effect, is limited by the relatively small shift in threshold voltage that can be obtained by doping (see, for example, Uchida and Matsumura, MRS Symp Proc. 149 247 (1989).